All the above posts give great advice..sometimes however it is just the way the plane is designed..You could have 2 Extras from 2 different designer's and maybe one guy stretched the tail a little or has the stab in a different location etc..On my WH Edge I balanced laterally, moved the CG around, reduced the weight..basically tried all the suggestions that everyone has given..Still had the rocking..I learned how to get rid of 80% of it but it takes a lot of minor corrections to keep it smooth..It can be done..it just takes some effort..BTW WH has re-designed the Edge to help with the rocking wings..I think the stab has been moved forward but Im not 100% sure..

OTOH, my EF 87" Yak has ZERO rocking at 45 deg 50..65 etc.. the only time the wings wobble is in a crosswind and even then it only takes minor rudder or aileron to stop it..